Recently, a DC-to-DC converter power supply for stabilizing the output voltage by controlling the pulse width has been widely used in electronic equipment. Furthermore, in order to achieve higher efficiency, also a synchronous rectification mode DC-to-DC converter power supply for reducing the loss of the forward voltage of a rectifying diode has been used in various cases in accordance with increased development of IC control circuits (see, for example, patent document 1: Japanese Patent Unexamined Publication No. 09-261950). FIG. 4 is a circuit diagram showing an example of a conventional DC-to-DC converter power supply; and FIG. 5 shows timing charts of main waveforms thereof. FIG. 4 shows an example of a case in which 3.3 V output and 1.8 V output are obtained from one DC input. Firstly, a 3.3 V output system is described.
When a DC voltage (for example, 5V to 10V of DC) is applied to DC input 1, oscillation and synchronization control circuit 30 that is a control IC starts to operate, further, drive circuit 5 is driven and P-channel MOS-FET 3 that is a switching element (hereinafter, abbreviated as MOS-FET 3) is driven. The drive waveform thereof is a voltage waveform at point k in FIG. 5 and the voltage at high level (time from t4 to t1) is substantially the same as that of DC input 1. Oscillation and synchronization control circuit 30 used herein is an IC having a special specification in which two kinds of drive pulses as shown in waveforms at points k and n in FIG. 5 are used with one-channel output, and these two drive pulses set dead time (which means a time when both two drives are turned OFF) in consideration of the rise time and the fall time of ON/OFF of MOS-FET to be driven respectively.
MOS-FET 3 is turned ON when gate voltage k is at low level (t1 to t4) and is turned OFF when gate voltage k is at high level (t4 to t1). Therefore, the output voltage of MOS-FET 3 is a voltage having a voltage waveform at point j in FIG. 5. This voltage is applied to coil 10. An electric current flowing in coil 10 during an ON period (t1 to t4) of MOS-FET 3 is an electric current having a current waveform at point m (time from t1 to t4) in FIG. 5. When an inductance value of coil 10 is small, the slope is steep and the peak value of the electric current becomes large. On the other hand, when an inductance value of coil 10 is large, the slope is gentle and the peak value of the electric current becomes small. In any case, it is necessary to select the inductance value of coil 10 so that the core of the coil is not saturated.
When MOS-FET 3 is turned OFF, the electric current flowing in coil 10 is not supplied, so that a counter electromotive force is generated at both ends of coil 10 and the potential at point j becomes negative and is clamped at the forward voltage of diode 9. As a result, energy accumulated in coil 10 becomes an electric current, and the electric current flows through loads (not shown) connected to capacitor 13 and first output 14 and diode 9. This electric current is called a reflux current and the loss thereof is reduced as a forward voltage of diode 9 is lower. Therefore, a schottky-barrier diode (hereinafter, referred to as SBD) is often used. In this case, however, the forward voltage is about 0.3V to 0.6V.
Therefore, during an ON period (t4 to t1) of diode 9, an element having a forward voltage lower than that of diode 9, that is, having a smaller loss is used to turn ON so as to allow a reflux current to bypass, thereby enabling the loss to be further reduced. This can be realized by forming a bypass circuit as follows. N-channel MOS-FET 32 that is a switching element (hereinafter, referred to as MOS-FET 32) is turned ON with the voltage waveform from t5 to t6 at point n by drive circuit 31. In general, in MOS-FET 32, voltage drop during an ON period can be expected to be 0.1V or less. Since it is lower than the forward voltage of diode 9 (0.3V to 0.6V), during the time, the reflux current flows through MOS-FET 32. This is described with reference to FIG. 5. The output waveform of drive circuit 31 is a voltage waveform at point n, and MOS-FET 32 is turned OFF at low level (t6 to t5). At this time, an electric current flows in diode 9 at the time from t4 to t5 and time from t6 to t1 as shown in the current waveform at point o. Furthermore, when the output of drive circuit 31 is at high level (t5 to t6), MOS-FET 32 is turned ON, and an electric current flows during the time from t5 to t6 as shown in the current waveform at point p.
Then, when looking at a portion at low level (t4 to t1) of the voltage waveform at point j, in the timings when diode 9 is ON, i.e., the timings from t4 to t5 and from t6 to t1, the voltage level of the forward voltage is between about −0.3V and −0.6V. On the other hand, in the timing when MOS-FET 32 is ON, i.e., the timing when an electric current is flowing in point p (t5 to t6), the voltage level is about −0.1V.
Then, by dividing and detecting the 3.3V output voltage with the use of resistors 11 and 12 and feeding back the voltage to oscillation and synchronization control circuit 30, the ON period of MOS-FET 3 is controlled and at the same time the ON period of MOS-FET 32 is controlled so as to carry out an operation so that the output is kept constant. Therefore, the shorter the period when an electric current flows in diode 9 is, the less the loss is and higher efficiency can be achieved. On the other hand, the ON period of MOS-FET 3 and the ON period of MOS-FET 32 coincide with each other, a large current flows, which may lead to destruction of the switching element. Therefore, care should be taken.
The basic operation of the 1.8V system output is the same as that of the 3.3V system output mentioned above, and thus the description therefor is omitted herein.
However, this conventional synchronous rectification mode DC-to-DC converter power supply, in which a plurality of outputs with different voltages are obtained from one input, has disadvantages that it is necessary to construct circuits respectively for each output system by using an oscillation and synchronization control circuit, a drive circuit and MOS-FET, and the like, thus increasing the circuit size. Another disadvantage is that in order to allow a plurality of drive circuits to be synchronized and controlled, it is necessary to use a special-purpose control IC as an oscillation and synchronization control circuit, thus increasing the cost.